1. Field of the Invention
The present invention relates to a fatty acid ester of cellulose and a process for the preparation thereof. More particularly, the present invention relates to a fatty acid ester of cellulose having excellent transparency, filterability and spinnability, and a process for the preparation thereof from a low-grade dissolving pulp as the raw cellulose material.
Furthermore, the present invention relates to cellulose diacetate and a process for the preparation thereof, more particularly, cellulose diacetate which has excellent filterability and hue, and yields few insolubles when dissolved in a solvent and in which the particle size of glucomannan acetate contained in the sediment formed by centrifuging is very small, and a process for the preparation thereof from, for example, a low-grade dissolving wood pulp.
2. Description of the Related Art
Among cellulose derivatives, fatty acid esters of cellulose, particularly cellulose acetate hold an important position industrially by virtue of their wide fields of applications including fibers for clothes, cigarette filters, plastics, film, coating materials, etc.
Although wood pulps are mainly used as the raw material for preparing fatty acid esters of cellulose, a highly purified high-grade dissolving pulp is particularly useful. This is because when the conventional, common process for the preparation thereof is applied to a low-grade wood dissolving pulp, a large amount of insoluble matters in a solvent, i.e., so-called insoluble residues are formed to impair its spinnability and transparency as a plastic, remarkably, so that filtration is required to secure these qualities.
The standard process for the preparation of cellulose acetate, which is a representative fatty acid ester of cellulose, basically comprises (1) a pretreatment step of disaggregating and disintegrating a starting pulp having a relatively high .alpha.-cellulose content, spraying the resulting pulp with acetic acid and mixing, (2) an acetylation step of reacting the pretreated pulp obtained in (1) with a mixed acid comprising acetic anhydride, acetic acid and an acetylation catalyst (e.g., sulfuric acid), (3) an aging step, i.e., a hydrolysis step, of hydrolyzing the obtained cellulose acetate to a desired degree of acetylation, and (4) a post-treatment step of precipitating the hydrolyed cellulose acetate, recovering the precipitates and subjecting the precipitates to purification, stabilization and drying, successively.
Improvement techniques for the above standard process from various standpoints have been reported in order to enable the preparation of a high-quality cellulose ester from a low-grade dissolving pulp which is available at a low cost.
U.S. Pat. No. 3,767,642 (patented on Oct. 23, 1973; Assignee: CELANESE Corp.) discloses that cellulose acetate having excellent transparency, filterability and spinnability is prepared by disintegrating a wood dissolving pulp having an .alpha.-cellulose content of 92 to 93% in a dilute aqueous solution of acetic acid to obtain a slurry, subjecting the slurry to a so-called slurry pretreatment wherein the removal of liquid and replacement of the liquid by acetic acid are repeated alternately, acetylating the resulting pulp according to the process of the prior art, neutralizing the sulfuric acid catalyst contained in the reaction system, and subjecting the reaction mixture to saponification and aging at 125 to 170.degree. C.
Further, U.S. Pat. No. 4,306,060 (patented on Dec. 15, 1981; Assignee: DAICEL CHEMICAL Ind., Ltd.) discloses a process for preparing cellulose acetate having excellent transparency, filterability and spinnability which comprises disintegrating a high-grade dissolving pulp having a high .alpha.-cellulose content by a common dry process, acetylating the resulting pulp in the presence of a small amount of sulfuric acid as the catalyst at high temperature to complete the acetylation in a short time, completely neutralizing the sulfuric acid catalyst contained in the system, and subjecting the reaction mixture to saponification at 110 to 120.degree. C. Further, it is also disclosed that this process enables the utilization of a low-grade dissolving pulp having a low .alpha.-cellulose content.
The characteristic common to the above techniques is that the saponification is conducted at high temperature. Such high-temperature saponification promotes the destruction and/or deformation of a hemicellulose acetate which otherwise damages the characteristics of cellulose acetate, whereby the hemicellulose acetate is converted into a harmless substance.
Attempts are disclosed to solve the above problem with attention being paid to the phenomenon that a low-grade wood dissolving pulp sheet is generally difficult to disintegrate due to its high density, which leads to the poor dispersibility of the pulp in the reaction medium to result in the formation of an insoluble residue. More specifically, there is disclosed a process wherein a pulp sheet is softened by wet disintegration to remove the heat generated in the disintegration as latent heat of evaporation, by which the thermal degradation of the pulp is inhibited to yield cellulose as a raw material having a rich reactivity, thus enabling the acetylation of the cellulose free from gel formation due to unreacted matters and insufficient reaction. Further, there is disclosed a process which comprises conducting the dry disintegration of a starting pulp in an improved disintegrator to thereby depress the deterioration of the reactivity of the pulp during disintegration to such an extent as not to have any effect on the acetylation, thus enabling the acetylation of cellulose free from the gel formation due to unreacted matters and insufficient reaction.
However, when the above-mentioned techniques are applied to the acetylation of a pulp having an .alpha.-cellulose content as especially low as 85 to 93%, the effect is too insufficient to avoid the lowering in the filterability of the resulting cellulose acetate.
Meanwhile, cellulose acetate is one of the organic acid esters of cellulose, the use of which is spread over many fields including the materials for clothes, cigarette filter tips, plastics, films, coating materials, medicines, foods, cosmetics and building materials and the production of which is the highest among cellulose derivatives, thus being industrially important.
The so-called acetic acid process using acetic anhydride as an acetylating agent, acetic acid as a diluent and sulfuric acid as a catalyst is a representative one as the industrial process for the preparation of cellulose diacetate. This process comprises (1) an acetylation step of pretreating a starting pulp having a relatively high .alpha.-cellulose content as the raw material by disaggregation, disintegration and spraying with acetic acid to activate the starting pulp and thereafter treating the activated pulp with a mixed acid comprising acetic anhydride, acetic acid and sulfuric acid to give cellulose triacetate; (2) an aging step, i.e., the hydrolysis step, of hydrolyzing the obtained cellulose triacetate into cellulose diacetate having a desired degree of acetylation; and (3) a post-treatment step of separating the cellulose diacetate from the reaction mixture by precipitation, followed by purification, stabilization and drying. The term "cellulose diacetate" used in this specification refers to one having a degree of acetylation of 50 to 57%. Further, the term "degree of acetylation" refers to a value as determined and calculated according to the method stipulated in ASTM-D-871, i.e., a value obtained by calculating the acetyl group content of cellulose acetate as the amount of acetic acid.
The cellulose diacetate prepared by the above process is again dissolved in a solvent such as acetone, molded and thereafter used as a commercial product.
Improvement techniques have been proposed from various standpoints to prepare a high-quality cellulose diacetate from a low-grade wood dissolving pulp having a low .alpha.-cellulose content through the above-mentioned essential steps.
Japanese Patent Publication-A No. 501/1987 (published on Jan. 6th, 1987) discloses that cellulose acetate having a very excellent hue can be prepared from a starting pulp having a low .alpha.-cellulose content for the preparation of viscose and cellulose ether by preliminarily swelling the pulp with a solvent.
Further, U.S. Pat. No. 3,846,403 (patented on Nov. 5th, 1974; Assignee: CANADIAN CELLULOSE CO.) discloses that in the preparation of a cellulose ester, a cellulose ester having characteristics not inferior to those prepared from a purer cellulose source is obtained by reacting a pulp with an esterifying agent other than an objective one, i.e., a lower alkanoic acid, to form hemicellulose ester prior to the reaction with the objective lower alkanoic anhydride. According to this technique, a high-quality cellulose ester can be obtained even from a starting pulp having a hemicellulose content far higher than 5%, i.e., as high as 8%, 10% or even above.
However, the technique described in the above Japanese Patent Publication-A No. 501/1987 is one for improving the hue of the cellulose acetate prepared from a low-grade wood dissolving pulp, so that it is silent upon the effect of improving other physical properties. Further, the technique has a defect that little effect is attained when a pulp having an .alpha.-cellulose content of 90% or below is used.
On the other hand, U.S. Pat. No. 3,846,403 discloses that even when the hemicellulose content of a starting pulp is 10% or above, the physical properties can be improved, and involves the description on the improvement in the filterability and the effect of reducing the weight of the insoluble residue with respect to cellulose triacetate, which is, however, different from the subject matter of the present invention, i.e., cellulose diacetate. Thus, it is unclear with respect to the properties of the cellulose diacetate.
Generally, a high-grade wood dissolving pulp having a high .alpha.-cellulose content is used as the raw cellulose material. However, obtaining high-grade pulp in quantity will become difficult because of the saving of resources on the global scale and the environmental pollution by pulp factories, so that the changeover to a low-grade wood dissolving pulp is believed to be unavoidable.
When cellulose diacetate is prepared from a low-grade wood dissolving pulp by the process of the prior art, the obtained cellulose diacetate produces a large amount of insolubles when dissolved in a solvent, which causes various problems such as lowering in the transparency of the product, an increase in the yellowness and a remarkable lowering in the characteristics in the filtration prior to the spinning step. There is a correlation between the .alpha.-cellulose content of a starting pulp and the properties of cellulose diacetate prepared from the pulp, so that when the adaptability to acetylation of a pulp is discussed only from the standpoint of chemical factors, the use of a starting pulp having an .alpha.-cellulose content of 93% or below gives only a very poor cellulose diacetate which is practically unusable as a commercial product owing to a remarkable increase in the amount of insolubles in a solvent such as acetone.
Various studies have been made on the above insolubles and abundant literature thereon can be found. Further, it is pointed out that the hemicellulose contained in a starting pulp may participate in the formation of the insolubles [see, e.g., Ueda et al., J. Japan Wood Research Soc., 34(4), 346-353 (1988)].
In order to prepare cellulose diacetate containing few insoluble components, therefore, it is thought necessary to employ either the means (1) of using a highly-purified, high-grade wood dissolving pulp having a low hemicellulose content as the raw material or the means (2) of reducing or removing the insolubles in the cellulose diacetate prepared from a low-grade dissolving pulp.
The former means, i.e., the use of a high-grade wood dissolving pulp as the raw material is actually industrially employed. However, the purification of a pulp by the removal of hemicellulose therefrom not only involves a lowering in the yield in preparing pulp from wood as the raw material to result in an increase in the production cost, but also is thought to be undesirable from the standpoint of the effective utilization of resources on the global scale. Further, even the content of solvent insolubles in the cellulose acetate prepared from a high-purity pulp cannot be said to be low enough, but is expected to be further lowered for an improvement in the quality.